1. Field of the Invention
The invention relates to a method of coating electrically conductive substrates, in which
(1) the electrically conductive substrate is immersed in an aqueous electrodip paint which contains at east one cathodically depositable synthetic resin and crosslinked polymer microparticles, PA0 (2) the substrate is connected as cathode, PA0 (3) a film is deposited by direct current on the substrate, PA0 (4) the coated substrate is removed from the electrodip paint and PA0 (5) the deposited paint film is baked. PA0 (A) a compound which contains on average more than 1.0 epoxide group per molecule, or a mixture of such compounds, and PA0 (B) a compound which contains not only a group reactive toward epoxide groups but also at least one ketimine or aldimine group in the molecule, or a mixture of such compounds
The invention also relates to aqueous electrodip paints, a process for the preparation of an aqueous dispersion of crosslinked polymer microparticles and dispersions prepared by this process.
2. Description of the Prior Art
The above-described cathodic electrodip coating method is known (cf. e.g. Glasurit-Handbuch Lacke und Farben, Curt R. Vincentz Verlag, Hanover 1984, pp. 374-384 and pp. 457-462) and has become established as a method for the automatic coating of large items such as wheels and, in particular car bodies.
EP-A 259,181 discloses a cathodic electrodip coating method in which electrodip paints are employed containing polymer microparticles which have a softening point at least 10.degree. C. above the bath temperature, a solubility parameter which differs by not more than 1.0 from the solubility parameter of the cathodically depositable synthetic resin, a refractive index which differs by 0.02 to 0.3 from the refractive index of the cathodically depositable synthetic resin, or have a crosslinking density of 0.01 to 5.05 mmol/g. The polymer microparticles employed are products prepared by polymerization of ethylenically unsaturated compounds. The polymer microparticles employed are intended in particular to improve edge covering.
EP-A-433,783 discloses a cathodic electrodip coating method in which electrodip paints are employed containing polymer microparticles comprising crosslinked urea-aldehyde, triazine-aldehyde or phenol-aldehyde resins or polymer particles of noncrosslinked (meth)acrylonitrile homo- or copolymers. In this case too the polymer microparticles employed are intended to improve edge covering.
The addition of the polymer microparticles described in EP-A 259,181 and EP-A-433,783 to electrodip paints leads in many cases to improvements in the edge covering. Despite the improved edge covering, the corrosion protection effect of the deposited electrodip coating films on the edge is inadequate. In addition, the addition of the polymer microparticles described in EP-A-259,181 and EP-A-433,783 has resulted in many cases, disadvantageously, in the occurrence of sedimentation in the electrodip paint, a worsening in adhesion to the substrate and/or to oversprayed coating layers, a sharp deterioration in the levelling and a reduction in the corrosion protection effect of the deposited electrodip coating films.
The present invention is based on the object of providing an improved cathodic electrodip coating method. In particular, this improved electrodip coating method should enable the production of coating films which cover the edges of the coated substrate well and which display at least some of the above-described disadvantages of the state of the art to a lesser extent or not at all.
This object is, surprisingly, achieved by a cathodic electrodip coating method of the type described above, which is characterized in that the crosslinked polymer microparticles contained in the aqueous electrodip paint can be prepared by reacting
to give a noncrosslinked intermediate product (C), which still contains on average at least one epoxide group per molecule, and dispersing this intermediate product in the presence of a Br onsted acid in an aqueous medium.
U.S. Pat. No. 4,017,438 describes noncrosslinked products of the reaction of polyepoxides and polyamine derivatives containing a secondary amino group and at least one ketimine group. These reaction products are employed as binders in electrodip paints. They do not bring about any improvement in the edge covering.
EP-A-351,141 describes crosslinked polymer microparticles which can also be employed in cathodically depositable electrodip paints. The polymer microparticles are prepared by dispersing a synthetic resin, which contains both hydrophilic groups and groups which make autocrosslinking possible, in an aqueous medium and then carrying out crosslinking of the dispersed polymer microparticles. The polymer microparticles employed in accordance with the invention are not described. In addition, the object on which the present invention is based is not addressed in EP-A-351,141. The incorporation of the polymer microparticles described in EP-A-351,141 in cathodically depositable electrodip paints leads to similar problems as with the addition of the polymer microparticles described in EP-A-259,181 and EP-A-433,783.
EP-A-421,238 describes crosslinked polymer microparticles which are prepared by reacting compounds containing epoxide groups with polyamines; the functionality of the compound containing epoxide groups should be greater than one and the functionality of the polyamines greater than two. The polymer microparticles employed in accordance with the invention are not described. The crosslinked polymer microparticles described in EP-A-421,238 are intended to be employed as a substitute for white pigments, in particular titanium dioxide. The object on which the present invention is based is not addressed in EP-A-421,238. The incorporation of the polymer microparticles described in EP-A-421,238 in cathodically depositable electrodip paints leads to similar problems as with the addition of the polymer microparticles described in EP-A-259,181 and EP-A-433,783.